Uneven subfloor

We are about to lay new ceramic tile over an existing vinyl floor. Under the vinyl is secure and stable plywood, however, the floor is uneven, with a large 3 tile deep dip inthe center of the floor. Can we use thinset to level it out, and how long should that dry before we begin laying tiles?

 

Steven,
To better answer any question, is to evaluate all the reasons for the problem. First of all I would be concerned why I had a 3/4" dip or hole in the floor. If possible, look at this area from your crawl space. You'll see then, what measures need to be taken. Next, (if nessary), cut the vinyl in this area to remedy the situation. This doesn't sound like the quick fix, but that's not what you're after. There is a lot of traffic in this area and you do not want to be reminded of what you should have done.
Now for the good news, if you believe the floor and vinyl is fine, then you have 3 options not in any order)1st: there is a product called 'level flow'. This is mixed and put on your floor and finds or seeks out to level your area. 2nd: thin set works well to accomplish the same result. Drying time will vary, but even so, you will not need to let it completely dry, as long as you can apply thin set with your notch trowel and not drag the underlayer.
And finally, you can butter your tile, which is, thin set applied to the back of your tile itself, with a combination of thin set properly applied to the floor. I hope this helps.